1. Field of the Invention
This invention relates, in general, to magnetic core structures for electrical inductive apparatus, such as transformers, and, more specifically, to magnetic core structures of the stacked type.
2. Description of the Prior Art
U.S. Pat. No. 3,153,215, which is assigned to the assignee of the present application, discloses magnetic core structures of the stacked type which have stepped-lap joints between the mitered ends of the leg and yoke portions of the magnetic core. In a stepped-lap joint, the joints between the mitered or diagonally cut ends of the leg and yoke laminations, in each layer of the lamination, are incrementally offset from similarly located joints in adjacent layers, in a predetermined stepped or progressive pattern, with the joints being stepped at least three times in one direction before the direction is changed or the pattern repeated. The stepped-lap joint was found to substantially improve the performance of the magnetic core, compared to magnetic cores which utilize conventional butt-lap type joints, by lowering the core losses, lowering the exciting volt-ampere requirements, and lowering the sound level of the magnetic core. In general, the prior art stepped-lap joint arrangements, as shown in U.S. Pat. Nos. 3,153,215; 3,477,053; 3,504,318 and 3,540,120, all of which are assigned to the assignee of the present application, obtain the desired stepped relationship between diagonally cut ends of the laminations by providing laminations for each leg or yoke portion which have the same longitudinal dimension between the diagonally cut ends. The stepped relationship is achieved by incrementally offsetting the midpoints of the laminations of any stacked group of laminations.
In prior art magnetic cores having stepped-lap joints, the stepped-lap joint between the inner leg and the top and bottom yoke laminations is constructed by forming a V-shaped notch in each of the top and bottom yoke laminations. The V-shaped notch in the yoke laminations is incrementally shifted, from layer to layer, parallel to the longitudinal axis of the magnetic core such that the inner leg laminations, which are of equal length, are also incrementally shifted parallel to the longitudinal axis or length of the magnetic core. In this manner, the equal length laminations of the top and bottom yokes are horizontally shifted from layer to layer which uniformly distributes the stepped-lap joint between the leg and yoke laminations and results in a symmetrical core structure which provides superior electrical characteristics. However, there is an inherent difficulty in constructing a horizontal stepped-lap magnetic core due to the multiple spaced end points of the inner leg laminations which are hidden from the view of the operator during assembly of the core thereby necessitating longer assembly times.
It is also known to step the inner leg laminations in a vertical direction, as shown in U.S. Pat. Nos. 3,153,215 and 3,743,991, both assigned to the assignee of the present application. In this type of magnetic core structure, the equal length inner leg laminations are vertically distributed, parallel to the straight side of the inner leg, by progressively notching one yoke lamination deeper and the other yoke lamination shallower than that of adjoining layers. Alternately, the length of the inner leg laminations may be incrementally varied from layer-to-layer to produce a vertical lap joint. In either vertical stepped-lap joint magnetic core structure, the equal length yoke laminations are incrementally shifted in a horizontal direction to form a stepped-lap joint with the leg laminations. It is also known to construct a stepped-lap joint with leg and yoke laminations that incrementally change lengths from layer-to-layer as shown in U.S. Pat. Nos. 3,670,279 and 3,918,153, both assigned to the assignee of the present application. As shown therein, the length of the leg and yoke laminations change in opposite directions from layer-to-layer. The midpoints of the laminations of each leg and yoke portion are aligned thereby incrementally offsetting the ends of the laminations from layer-to-layer to form the desired stepped-lap pattern. This type of magnetic core structure has significantly lower core losses and noise levels compared to magnetic core structures having stepped-lap joints formed by incrementally shifted, equal length leg and yoke laminations.
It would be desirable to provide a magnetic core having stepped-lap joints which exhibits lower losses than prior art magnetic core structures. It would also be desirable to provide a three phase magnetic core having vertical stepped-lap joints between the inner leg and the top and bottom yoke laminations which is symmetrical about the inner leg laminations. Also, it would be desirable to provide a three phase magnetic core formed of unequal length leg and yoke laminations which have their midpoints aligned so as to offset the ends from layer-to-layer. Finally, it would be desirable to provide a three-phase magnetic core formed of unequal length leg and yoke laminations which has a reduced amount of voids between the yokes and the leg laminations and, further, in which the voids are uniformly distributed between the outer leg and the yoke laminations in each group of layers of the laminations.